Analyses of load transmission through the lumbar spine are used to set safe limits for manual handling occupations to prevent low back pain and spinal injuries, and to guide rehabilitation of people with low back pain. This proposed research addresses the need for an improved method of analyzing forces transmitted through the human lumbar spine. This model will extend and improve previous models by the addition of three important elements: (1) the model will be truly three-dimensional with a realistic representation of muscular anatomy accounting for muscles which cross several motion segments. (2) It will respect the need for the lumbar spinal column to be loaded in a way which is structurally stable. (3) The spinal motion segments will be represented as realistic flexible structures. This model will be developed and validated through five logical steps by incorporating previously published data and using new modeling tools to examine plausible muscle force distributions and their contributions to spinal stability. Based on preliminary studies it is expected that the model will demonstrate how antagonistic muscles of the trunk are recruited. Preliminary results suggest that this may occur under three circumstances: (1) to maintain equilibrium at multiple levels of the spinal column; (2) to minimize muscles stresses, and (3) to maximize the structural stability of the spinal column. The model will also be used to show how different spinal postures and certain spinal injuries affect its stability and load-bearing capacity. The results will be used to identify loading conditions, spinal configurations and muscular activation patterns which may place the spine at risk for 'self-injury'. The methods developed in this study are also expected to have wider applicability to understanding human joint biomechanics and stability (e.g. the pinch grip between two digits.)